DOC: skipna not an argument of drop_duplicates

From what I can tell, `skipna` keyword was both added and subsequently
removed in #1244. However, the docstring for DataFrame.drop_duplicates
still lists it as a parameter.

Author: ivanov

pandas/core/frame.py

@@ -2578,8 +2578,6 @@ def drop_duplicates(self, cols=None, take_last=False, inplace=False):
             default use all of the columns
         take_last : boolean, default False
             Take the last observed row in a row. Defaults to the first row
-        skipna : boolean, default True
-            If True then keep NaN
         inplace : boolean, default False
             Whether to drop duplicates in place or to return a copy